Rechargeable cells that use a nickel hydroxide positive electrode and a metal hydride forming hydrogen storage negative electrode ("metal hydride cells") are known in art.
Generally, when an electrical potential is applied between the electrolyte and a metal hydride electrode in a metal hydride cell, the negative electrode material (M) is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of a hydroxyl ion; upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron: ##STR1##
The reactions that take place at the positive electrode of a nickel metal hydride cell are also reversible. Most metal hydride cells use a nickel hydroxide positive electrode. The following charge and discharge reactions take place at a nickel hydroxide positive electrode: ##STR2## In a metal hydride cell having a nickel hydroxide positive electrode and a hydrogen storage negative electrode, the electrodes are typically separated by a non-woven, nylon or polypropylene separator. The electrolyte is usually an alkaline aqueous electrolyte, for example, 30 weight percent potassium hydroxide.
The first hydrogen storage alloys to be investigated as battery electrode materials were TiNi and LaNi.sub.5. Many years were spent in studying these simple binary intermetallics because they were known to have the proper hydrogen bond strength for use in electrochemical applications. Despite extensive efforts, however, researchers found these intermetallics to be extremely unstable and of marginal electrochemical value due to a variety of deleterious effects such as slow discharge and poor cycle life brought about by oxidation, corrosion, poor kinetics, and poor catalysis. These simple alloys for battery applications reflect the traditional bias of battery developers toward the use of single element couples of crystalline materials such as NiCd, NaS, LiMS, ZnBr, NiFe, NiZn, and Pb-acid. In order to improve the electrochemical properties of the binary intermetallics while maintaining the hydrogen storage efficiency, early workers began modifying TiNi and LaNi.sub.5 based alloys.
In U.S. Pat. No. 4,623,597 (the '597 patent), the contents of which are incorporated by reference, one of the present inventors, Ovshinsky, described disordered multicomponent materials for use as negative electrodes in electrochemical cells for the first time. In this patent, Ovshinsky describes how disordered materials can be tailor made to greatly increase hydrogen storage and reversibility characteristics. Such disordered materials are amorphous, microcrystalline, and/or polycrystalline (lacking long range compositional order). The framework of active materials of these disordered materials consist of a host matrix of one or more elements and modifiers incorporated into this host matrix. The modifiers enhance the disorder of the resulting materials and thus create a greater number and spectrum of catalytically active sites and hydrogen storage sites. Multiorbital modifiers provide a greatly increased number of storage sites due to various bonding configurations, orbital overlap, and hence a spectrum of bonding sites. Due to the different degrees of orbital overlap and the disordered structure, an insignificant amount of structural degradation occurs during charge/discharge cycles or rest periods between charge/discharge cycles resulting in long cycle life and shelf life.
The '597 patent marks the beginning of Ovshinsky's modification program of TiNi and LaNi.sub.5 based alloys at Energy Conversion Devices (ECD) of Troy, Mich. The present invention represents a departure by Ovshinsky and his team and the development of a new family of alloys.
The '597 patent describes disordered battery materials produced from Ti-Ni and Mg-Ni. The described Mg-Ni disordered materials had a capacity as high as 566 mAh/g. However, a number of practical problems, such as the metallurgical differences between Mg and Ni made further development and fabrication of these materials particularly difficult at the time. Thus, the pioneering principles described in the '597 patent were initially applied to Ti-Ni based materials. Subsequently, these evolved into Ti-V-Zr-Ni type active materials such as disclosed in U.S. Pat. No. 4,551,400 ("the '400 Patent") to Sapru, Hong, Fetcenko, and Venkatesan, the disclosure of which are incorporated by reference.